Mica paper and method of preparing it



United States Patent" 2,914,107 MICA PAPER AND METHOD OF PREPARING IT George L. Gaines, Jr., 'Scotia, N.Y., assignor to General Electric Company, a corporation of New York No Drawing. Application September 24, 1956 Serial No. 611,773

2 Claims. (Cl. 1542.6)

This invention relates to mica paper. More particularly, this invention relates to sheets of mica paper which are characterized by improved tensile strength and im-' proved moisture resistance and to the process by which these improved products-are obtained.

Heretofore, the preparation of mica paper has been known in the art. For example, mica paper preparation is described in Patents 2,549,880-Bardet, 2,614,055-de Senarclens and 2,709,158-Bouchet. Mica paper prepared by the methods of these patents as well as by other known methods is gaining commercial acceptance because of the excellent electrical properties of the mica paper at low temperatures and because of its chemical inertness at elevated temperatures. However, the present mica paper is lacking in the high tensile strength and high moisture resistance desirable in such a product.

' It is an object of the present invention to provide an improved mica paper which is characterized by having a relatively high tensile strength and by being relatively resistant to the effect of moisture.

This and other objects of my invention are accomplished by subjecting comminuted mica particles to the action of an aqueous potassium silicate solution.

The term mica paper as used in this application is used in its usual sense to refer to a sheet-like aggregate of mica particles which has been prepared by grinding up naturally occurring mica flakes, dispersing the resulting fine particles in an aqueous solution and then forming a sheet of mica paper from this aqueous slurry by conventional paper making techniques.

The mica paper of the present invention differs from that of the prior art in that the mica particles have been subjected to the action of an aqueous potassium silicate solution before being formed into the final sheet.

The first step in the preparation of the mica paper of the present invention is the grinding up of naturally occurring mica flakes into extremely fine particles. These mica flakes may comprise phlogophite, lepidolite, or preferably muscovite. The grinding of the mica flakes may be accomplished by any suitable method. In general, the grinding up is accomplished by first heating the mica flakes at a temperature of about 800 C., e.g., from about 700 to 900 C., and preferably from 775 to 825 C., for

, a time of about ten minutes, e.g., from about 5 to 20 minutes. This heating step causes a loss in weight of the mica equal to about 2 percent of the original weight of the mica. The effect of this heating step is to soften the mica flakes While at the same time delaminating and increasing the bulk volume of the mica. This softening and delaminating tends to facilitate the subsequent grinding of the mica flakes into fine particles. However, it should be understood that the mica flakes may be ground up into fine particles without first heat treating the particles.

The mica flakes, which may or may not have been heat treated or fired, are then added to an aqueous medium and agitated by any suitable device, such as a high speed comminuter or mixer, to convert the flakes into small particles or platelets. The comminution of the mica can be carried out in an aqueous suspension containing from about 0.1 to 10 percent and preferably about 1 percent by weight of mica. This results in a pulp-like suspension of mica in which the particle size of the mica flakes has a wide distribution. The extra-fine and extra-coarse particles in this comminuted aqueous suspension may then be removed and the resulting slurry is then treated with an aqueous potassium silicate solution. These aqueous potassium silicate solutions are well known in the art and many of these solutions are described, for example, in the book Soluble Silicates, James G. Vail, Reinhold Publishing Company, New York (1952). These solutions are generally prepared by firing a mixture of potassium oxide and silica to form a potassium glass. This glass is then dissolved in hot water to obtain the desired potassium silicate solution. Potassium silicate solutions vary in the ratio of the potassium oxide to silica contained therein and in the percentage of solids contained therein. In the practice of our invention we employ potassium silicate solutions in which the ratio of K 0 to SiO varies from about 1:1.5 to about 1:4. Preferably, the potassium silicate solutions contain one mole of potassium oxide per three to four moles of silica. The aqueous solutions of the potassium silicate may contain from about 40 to 80 and preferably 50 to 75 percent by weight of water.

In preparing the potassium silicate treated mica paper of the present invention, a mica slurry in water is prepared Which contains from about 0.5 to 5 and preferably about 1 to 2 parts by weight of mica per hundred parts of slurry. The aqueous potassium silicate solution is then added to this slurry and the resulting silicate-com taining slurry is allowed to stand for times which can vary from a few seconds up to several days. However, complete treatment of the mica particles in the slurry is accomplished during the first several seconds of the contact of the mica particles with the silicate-containing solution. The amount of the silicate solution which is added to the aqueous mica slurry may vary within wide limits. It has been found that satisfactory treatment of the mica flakes is accomplished when the silicate solution is added in sufiicient quantity to provide about 10 to 50 and preferably 20 to 30 parts by weight of the silicate solution per part of mica in the slurry.

In addition to treating the mica flakes by the method of the preceding paragraph, the treatment of the mica with the potassium silicate solution may also be accomplished by employing a potassium silicate solution as the medium in which the large mica flakes are ground up into fine mica particles. Thus, the same solution will be effective to facilitate the comminution of the mica flakes and the treatment of the comminuted mica flakes with potassium silicate.

Regardless of how the mica particles are treated with the potassium silicate, the resulting mixture comprises mica in an aqueous potassium silicate solution. This slurry is then formed into paper-like sheets by conventional paper making techniques. Thus, the slurry is dispersed on top of a screen and the liquid medium is drawn through this screen, leaving a wet sheet of mica particles on the surface of the screen. The wet mica sheet can then be dried by evaporation at room temperature or elevated temperatures. Where elevated temperatures are employed, it is desirable to employ temperatures of from about 150 (3., since this temperature has been found satisfactory to remove all of the liquid from the sheet in a relatively short time. After removal of the moisture, the resulting sheet is fairly rigid and is resistant to both thermal deterioration at elevated temperatures and disintegration upon contact with water.

In addition to forming the final mica sheet'by evapora- 3 tion of the suspension medium from the particles, satisfactory mica paper is also obtained by calendering the damp mica sheet between conventional calendering rolls. This calendering operation may be carried out at a temperature of from about 110-175 C. to facilitate the removal of moisture from the sheet.

In place of calendering, the mica sheet on the paper making machine screen may be dried by heating at an elevated temperature and may then be pressed in a suitable high pressure apparatus, for example, at a pressure of from 200 to 1000 p.s.i. to smooth out any rough portions of the sheet.

The fact that the potassium silicate treated mica paper of the present invention has both a higher tensile strength and is more resistant to moisture than a conventional sheet of mica paper prepared without any additive is entirely unexpected and unpredictable in view of the fact that when mica sheets are treated with either tetraethyl silicate or an aqueous sodium silicate solution, the resulting sheet disintegrates completely upon contact with water as if the sheet had not been treated at all.

The following examples are illustrative of the practice of my invention and are not intended for purposes of limitation. In each of the examples a slurry of finely divided mica particles in water was employed. The slurry was formed by firing Bengal ruby muscovite at a temperature of about 800 C. for about 10 minutes, at which time there had been a weight loss of about 2 percent by weight of the original weight of the mica. After cooling, this mica was then added to distilled water and the resulting slurry was violently agitated to comminute the mica particle The extra-fine and extra-coarse particles of mica in the slurry were removed. All parts mentioned in the examples are parts by weight.

Example 1 One hundred parts of a mica slurry containing 1 part of mica was prepared by the method previously described. To this slurry was added various numbers of parts of a potassium silicate solution containing 67 percent by weight of water and containing 1 mole of potassium oxide per 3.9 moles of silica. The table below lists the tensile strengths observed in mica sheets having a thickness of about 4 to 5 mils, which were prepared by adding the potassium silicate solution to the mica slurry, allowing the resulting slurry to stand for 1 to 18 hours, dispersing the slurry on top of filter paper, removing the water from the slurry, pressing the resulting sheets at 500 p.s.i. and drying them at 110 C. The table below lists the number of parts of the potassium silicate solution employed and the resulting tensile strengths.

Parts potassium silicate: Tensile strength, p.s.i.

None (control) 1700 3800 20 4300 40 7200 The moisture resistance of the sheets of mica paper prepared in this example was examined by dipping each of the sheets in a vessel of water. When the control was dipped in this vessel, the sheet immediately disintegrated and formed a slurry of mica particles in water. When the potassium silicate treated sheets were dipped in the water, there was no noticeable deterioration of the sheet at the end of 2 minutes.

Example 2 pletely disintegrated on contact with water while the potassium silicate treated sheet was not affected by water.

Example 3 The procedure of Example 1 was repeated except that the potassium silicate solution contained 61 percent by weight of water and contained 1 mole of potassium oxide for every 3.3 moles of silica. The tensile strength of a sheet treated with 20 parts of this potassium silicate solution was 3500 p.s.i. as compared with the 1900 p.s.i. tensile strength observed in the control.

Example 4 The procedure of Example 1 was repeated except that the potassium silicate solution contained 73 percent by weight of water and contained 1 mole of potassium oxide for every 3.9 moles of silica. The sheet formed in this method had a tensile strength of about 4500 p.s.i. as compared with about 2000 p.s.i. for a control. Twenty parts of the potassium silicate solution were employed. When the procedure of this example was repeated except that 20 parts of a sodium silicate containing 67 percent by weight of water and containing 1 mole of sodium oxide per 3.9 moles of silica, the resulting sodium silicate treated sheet had a tensile strength of about 1500 p.s.i., as compared with the 2000 p.s.i. tensile strength of the control. Both the control and the sodium silicate treated sheets disintegrated completely on contact with water.

Although the exact mechanism by which the potassium silicate solutions strengthen and improve the moisture resistance of the mica paper of the present invention 15 not known, it has been found that mica paper prepared by the process of the present invention is slightly heavier than mica paper prepared from the same amount of mica slurry which does not also include the potassium silicate solutions. In general, the increase in Weight of the mica sheet is about 5-15 percent.

t is obvious that the process of the present invention may be carried out by methods difierent mechanically from the methods specifically illustrated in the examples.

Thus, the dry mica flakes may be added to a solution of potassium silicate which has already been diluted withwater to provide the amount of water desired in the resulting potassium silicate treated slurry. When this is done, the mica flakes are merely added to the diluted potassium silicate solution and the diluted potassium silicate solution is subjected to violent agitation to grind up the mica flakes into mica particles. The extra-fine and extra-coarse particles may be removed from this solution and a sheet formed from the resulting slurry. The liquid recovered during the formation of the sheet may be reused to treat additional mica flakes. When the solution is reused, additional potassium silicate solution is added to the diluted solution to bring the potassium silicate concentration up to the desired value.

Although the foregoing examples have described only a limited number of mica slurry concentrations and a limited number of potassium silicate solutions, it should be understood that both the concentration of the mica slurry and the characteristics of the potassium silicate solution may be varied within extremely wide limits without departing from the scope of the present invention.

The mica paper prepared by the method of this invention has the same utility as other mica paper. Thus, the mica paper can be employed as slot insulation in dynamoelectric machines, can be cut into tape and employed as insulation for electrical conductors, can be employed as electrical and thermal insulation in electron tubes, etc. The use of the mica paper of the present invention is particularly desirable where the mica paper is liable to come into contact with moisture, since the mica paper of this invention is resistant to the disintegration which usually accompanies contact of mica paper with water.

5 6 What I claim as new and desire to secure by Letters at a temperature up to about 150 C. to obtain a sheet Patent of the United States is: which has a good tensile strength and good moisture 1. The method of preparing mica paper of improved resistance. tensile strength and moisture resistance which consists The Product pr p r d by t e t od 0f Claim 1- essentially of (1) contacting an aqueous slurry of finely 5 References Cited in the file of this patent commmuted mica particles w1th an aqueous potassium silicate solution containing from 1.5 to 4 moles of silica UNITED STATES PATENTS per mole of potassium oxide, (2) forming a wet paper- 1,872,234 Boughton Aug. 16, 1932 like sheet from said potassium silicate treated aqueous 2,493,693 Parkinson Jan. 3, 1950 slurry of finely divided mica, and 3 drying said sheet 2,493,694 Shepard Jan. 3, 1950 

1. THE METHOD OF PREPARING MICA PAPER OF IMPROVED TENSILE STRENGTH AND MOISTURE RESISTANCE WHICH CONSISTS ESSENTIALLY OF (1) CONTACTING AN AQUEOUS SLURRY OF FINELY COMMINUTED MICA PARTICLES WITH AN AQUEOUS POTASSIUM SILICATE SOLUTION CONTAINING FROM 1.5 TO 4 MOLES OF SILICA PER MOLE OF POTASSIUM OXIDE, (2) FORMING A WET PAPERLIKE SHEET FROM SAID POTASSIUM SILICATE TREATED AQUEOUS SLURRY OF FINELY DIVIDED MICA, AND (3) DRYING SAID SHEET AT A TEMPERATURE UP TO ABOUT 150*C. TO OBTAIN A SHEET WHICH HAS A GOOD TENSILE STRENGTH AND GOOD MOISTURE RESISTANCE. 